When a semiconductor device is manufactured, various heat processing apparatuses are used for subjecting a semiconductor wafer, which is an object to be processed, to processes such as an oxidation process, a diffusion process, and a CVD (Chemical Vapor Deposition) process. A general heat processing apparatus includes a heat processing furnace which is mainly composed of: a processing vessel (reaction tube) capable of accommodating semiconductor wafers and heat-processing the same; and a heater (heating device) that is disposed to cover a circumference of the processing vessel, for heating wafers in the processing vessel. The heater is mainly composed of a cylindrical heat insulating member, and a heating resistor disposed on an inner circumference of the heat insulating member via a supporting member.
In a heat processing apparatus capable of performing a batch process, for example, there is used, as the heating resistor, a helical heating wire (heating resistance wire) arranged along an inner wall surface of the cylindrical heat insulating member. The heating wire can heat an inside of the furnace to a high temperature such as about 800° C. to 1000° C. As the heat insulating member, there is used a member that is formed by burning a heat insulating material, such as ceramic fibers, into a cylindrical shape. The heat insulating member can reduce a heat quantity lost as radiant heat and conductive heat, so as to enhance efficiency in heating. As the supporting member, there is used a ceramic member, for example. The ceramic supporting member can support the heating wire at predetermined pitches, while allowing heat expansion and heat shrinkage of the heating wire.
In the above heat processing furnace, in order that the heating wire, which is helically formed, can be thermally expanded and thermally shrunk, the heating wire is supported such that a clearance is defined between the heating wire and the heat insulating member. By using the heating wire under a high temperature, the heating wire undergoes a creep strain, and slowly increases in length over time. In addition, the heating wire is thermally expanded during a heating operation. On the other hand, there is an apparatus that quickly cools a heating wire by blowing air thereto so as to decrease a temperature. Due to the repeated rise and drop in temperature, the heating wire is likely to be deformed. This may generate a short-circuit between adjacent parts of the deformed heating wire, which may invite disconnection.
Particularly in a vertical-type heat processing furnace, the heating wire is moved in the supporting member because of the repeated heat expansion and heat shrinkage caused by the rinse and drop in temperature, and the heating wire is moved downward little by little because of gravitation. Then, the moving amount is accumulated at a lowermost turn of the heating wire. Namely, due to the accumulation of the movement of the heating wire, a winding diameter of the lowermost turn is increased. When the heating wire of the increased winding diameter reaches an inner surface of the heat insulating member and cannot be expanded outside any more, the heating wire is then deformed in the up and down direction. Thus, there is a possibility that a short-circuit occurs between a part and another part adjacent thereto of the heating wire, resulting in some disconnection.
In order to eliminate these problems, the following structure has been proposed (see, JP10-233277A). Namely, in order to prevent such an accumulation to one side of the elongated heating wire caused by creep and thermal expansion or the like, a fixing member of a rod-like shape is attached to an outside portion of the heating wire by welding, and a distal end of the fixing member is buried to be fixed in a heat insulating member, so that the fixing member projects outward in a radial direction of the furnace.
However, in the above structure in which the fixing member is merely joined to the outside portion of the heating wire by welding, the joined portion is exposed to a high temperature. In addition, it can be considered that a stress tends to concentrate on the joined portion when the heating resistor is thermally expanded or thermally shrunk, which entails deterioration in durability (reduction in lifetime) of the heating wire. Further, since the fixing member has a bar-like shape, the fixing member may easily drop out of the heat insulating member, whereby it is difficult to secure a sufficient holding force for the fixing member.
Moreover, when it is desired to quickly increase or decrease the temperature of a wafer, a large power has to be applied to a heating wire during the quick temperature-increasing operation. However, a conventional, general heating wire may not withstand the large load, and may be prone to be disconnected. For this reason, such a large power cannot be actually applied, and thus the quick temperature increase/decrease operation is limited. Although the use of a heating wire resistant to disconnection can overcome the difficulty, this incurs increase in cost because such a heating wire is expensive.
Meanwhile, in order to make longer the lifetime (to improve the durability) of the heating wire by reducing a load applied thereto, it is effective to increase a ratio of a surface area of the heating wire (element surface area) relative to a supplied power. This is because, when the heating wire surface area is increased, a surface temperature of the heating wire is lowered, to thereby reduce a load of the heating wire. Since a so-called spiral (helical)-type heating wire is suitable as a design for load reduction and can be efficiently arranged in a desired space, such a heating wire is widely used.
However, as shown in FIG. 9, for example, a heater or a heat processing furnace using a spiral-type heating wire conventionally employs a structure in which a heating wire 18 is buried in a heat insulating member 16 so as to fix the heating wire 18. Thus, an object to be heated in a reactor core is heated via the heat insulating member 16, so that it is difficult to quickly increase a temperature of the object. It is also difficult to quickly decrease the temperature of the object, because the heating wire 18 is cooled via the heat insulating member 16, in addition to an effect of increase in heat capacity caused by the heat insulating member 16. Moreover, since there is no clearance for allowing expansion of the heating wire 18, the heating wire 18 itself is stressed when it is expanded. Thus, durability of the heating wire may be not sufficient.